Internet DRAFT - draft-ietf-sip-hop-limit-diagnostics
draft-ietf-sip-hop-limit-diagnostics
SIP WG S. Lawrence
Internet-Draft Pingtel Corp.
Updates: 3261 (if approved) A. Hawrylyshen
Expires: December 18, 2006 Ditech Networks Inc.
R. Sparks
Estacado Systems
June 16, 2006
Diagnostic Responses for Session Initiation Protocol Hop Limit Errors
draft-ietf-sip-hop-limit-diagnostics-03
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Session Initiation Protocol (SIP) imposes a limit on the number
of hops a request can transit on the way to its destination. When
this limit is reached, a 483 (Too Many Hops) error response is
returned. The present form of the 483 response does not provide
enough information for the UAC or proxy on the path to diagnose
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failures whose symptom is that the hop limit is reached. This
document specifies additional diagnostic information to be returned
in a 483 response.
Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
2. Diagnosing Hop Limit Exceeded Failures . . . . . . . . . . . . 4
2.1. Limitations of the 483 Error Response . . . . . . . . . . 4
2.2. Improved Diagnostic Information in Responses . . . . . . . 5
3. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Pruning Responses . . . . . . . . . . . . . . . . . . . . 7
4. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
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2. Diagnosing Hop Limit Exceeded Failures
The SIP protocol imposes a limit on the number of hops a request can
transit on the way to its destination. The number of hops remaining
for the request is carried in the Max-Forwards header, and is
decremented each time the request is forwarded. When a SIP User
Agent receives a request whose Max-Forwards is zero (0), it returns a
483 error response to indicate that the limit was reached.
The 483 response alone does not provide enough information for the
originating UAC to determine where the problem lies. The problem is
rarely that the target of the request was actually further away than
the Max-Forwards limit allowed. The problem is usually incorrect
routing; often a routing loop.
2.1. Limitations of the 483 Error Response
Section 20.22 of RFC 3261 [RFC3261] says:
The Max-Forwards header field must be used with any SIP method to
limit the number of proxies or gateways that can forward the
request to the next downstream server. This can also be useful
when the client is attempting to trace a request chain that
appears to be failing or looping in mid-chain.
In practice, there is too little information returned in a 483
response for it to be of much use as a diagnostic tool. When a
request has traversed a series of proxies, the response follows the
Vias back to the requester - in the case of a typical 483 response it
can be difficult to determine even what server the response came
from. Even when the rejecting server does identify itself, it can be
difficult to figure out why the request got there.
The following is an actual example request; the IP addresses and
domain names have been changed, but it is otherwise complete (it was
intentionally sent without SDP for brevity):
INVITE sip:9999@example.com SIP/2.0
Via: SIP/2.0/TCP 10.1.1.20:59449
;branch=z9hG4bK-56ec69968c31f498c9a5573a00c8fc04
To: sip:9999@example.com
From: Sip Send <sip:sipsend@10.1.1.20>;tag=08e2f515
Call-ID: 159213b1aa5a67bc6eca6c4c2bad9f94@10.1.1.20
Cseq: 1 INVITE
Max-Forwards: 1
User-Agent: sipsend/0.02
Date: Wed, 12 Oct 2005 20:09:29 GMT
Content-Length: 0
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This request was sent with the Max-Forwards header field value set to
only 1 to force the error response: it should traverse only the first
outbound proxy, and then be rejected by the next system that it
encounters.
The response received in this case was:
SIP/2.0 483 Too Many Hops
Via: SIP/2.0/TCP 10.1.1.20:59449
;branch=z9hG4bK-56ec69968c31f498c9a5573a00c8fc04
To: sip:9999@example.com;tag=-1574266585
From: Sip Send <sip:sipsend@10.1.1.20>;tag=08e2f515
Call-ID: 159213b1aa5a67bc6eca6c4c2bad9f94@10.1.1.20
Cseq: 1 INVITE
Content-Length: 0
There is no indication in the response of what server returned the
error. Even with the error only one hop beyond the first proxy,
there is no way to determine if that first proxy has routed the
request incorrectly.
2.2. Improved Diagnostic Information in Responses
In some ways, the SIP Max-Forwards mechanism is analogous to the Time
To Live (TTL) field in an IP datagram. The TTL field was originally
[RFC0791] intended to be the maximum number of seconds that a
datagram should remain in the network. In practice, IP TTL has
evolved into a hop count, since each system forwarding a datagram was
(is) required to decrement the TTL by (at least) one. As an aid to
diagnosing problems, the Internet Control Message Protocol [RFC0792]
defines a "Time Exceeded Message" to be sent by any system that
discards an IP datagram because it was received with a TTL value of
zero (0). The Time Exceeded Message is sent to the source address of
the discarded datagram, and includes a field that carries the
"Internet Header + 64 bits of Original Data Datagram". This allows
the originator to see the datagram as it appeared where it was
discarded. The 'traceroute' tool determines the route followed
between a given pair of IP addresses by sending a series of IP
packets from the source to the destination with gradually increasing
TTL values. As each packet reaches its limit, an ICMP Time Exceeded
Message is returned by the router that is discarding it; some checks
on the route can be made by examining the original packet as it
arrived at each hop.
As an aid to diagnosing problems that result in 483 responses, it
would be useful to know how the failed request arrived at the
rejecting system; both what path it followed to get there, and what
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the request looked like when it ran out of hops. One way to
accomplish this is to return the SIP header of the rejected message
to the UAC that originated it. Doing so is already allowed by
existing rules:
RFC 3261 [RFC3420] (section 7.4) says: "All responses MAY include
a body.".
RFC 3420 [RFC3420] defines the Content-Type "message/sipfrag" to
"allow SIP entities to make assertions about a subset of a SIP
message".
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3. Proxy Behavior
This document adds the following new rule for all SIP Proxy
implementations:
Any 483 response SHOULD be constructed with both:
A message body of type message/sipfrag containing as much as
possible of the SIP header from the rejected request.
A Warning header with a warn-code of 399 that identifies the
system returning the error.
Exceptions to this are allowed so that a system is allowed to omit
parts of the message either to limit the size of the response or
to conform to a local security policy. See Section 3.1.
3.1. Pruning Responses
A server may be unable or unwilling to return the full request
message in every 483 response. The returned message may exceed the
maximum message size it can handle, or may include security-sensitive
information.
It is RECOMMENDED that when the complete message cannot be returned,
that at least all of the Route and Via headers be included in the
message/sipfrag body. In the example (Section 6), this would at
least enable the end user to determine which proxies were in the
routing loop and how the request arrived there, but not the specific
address transformations that caused the loop.
If including all Via and Route headers is still too large, the
implementation SHOULD remove the oldest Vias (those nearest the
message originator) until the size is acceptable; the only exception
to this rule is when . This way, the originator can detect that some
Vias were removed (because the one that it put on is missing).
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4. UAS Behavior
Since a UAS is not required to validate the Max-Forwards header field
value when processing a received message, it is not required to
return the received message header as described above for proxies,
but it MAY do so.
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5. UAC Behavior
This specification does not mandate any new behavior for a UAC. The
returned message is available to the UAC to either pass on to the
user or to use for any automated diagnostic process.
Note that a UAC MUST be prepared to receive message bodies in a
response that it does not understand and did not request; this is
already required by [RFC3261] section 20.1 Accept:
The Accept header field follows the syntax defined in [H14.1].
The semantics are also identical, with the exception that if no
Accept header field is present, the server SHOULD assume a default
value of application/sdp.
[H14.1] refers to RFC 2616 [RFC2616], which specifically limits the
semantics of the Accept header fields in section 10.4.7 '406 Not
Acceptable' as follows:
Note: HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept headers sent in the
request. In some cases, this may even be preferable to sending a
406 response. User agents are encouraged to inspect the headers
of an incoming response to determine if it is acceptable.
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6. Example
This example shows how this proposed change is used to diagnose an
example routing problem.
Here is a request sent to a proxy that implements the suggested
content in a 483 response.
INVITE sip:9999@example.com SIP/2.0
Via: SIP/2.0/TCP 10.1.1.20:40221
;branch=z9hG4bK-931ea14405e9da8c95cf4ed60a71f59f
To: sip:9999@example.com
From: Sip Send <sip:sipsend@10.1.1.20>;tag=612f37e7
Call-ID: 7a26fdad2cb40d48e81e10d6fce39825@10.1.1.20
Cseq: 1 INVITE
Max-Forwards: 9
User-Agent: sipsend/0.02
Date: Fri, 14 Oct 2005 15:35:53 GMT
Content-Length: 0
The target user '9999' is one that has been deliberately configured
to go into a forwarding loop alternating between two addresses
(neither of them the original target); a situation that is currently
difficult to diagnose. A relatively low Max-Forwards header field
value of 9 was chosen to improve readability.
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The response received was:
SIP/2.0 483 Too many hops
Warning: 399 192.0.2.162:5080 Too Many Hops
From: Sip Send <sip:sipsend@10.1.1.20>;tag=612f37e7
To: sip:9999@example.com
Call-Id: 7a26fdad2cb40d48e81e10d6fce39825@10.1.1.20
Cseq: 1 INVITE
Via: SIP/2.0/TCP 10.1.1.20:40221
;branch=z9hG4bK-931ea14405e9da8c95cf4ed60a71f59f
Content-Type: message/sipfrag
Content-Length: 1014
Date: Fri, 14 Oct 2005 15:27:47 GMT
INVITE sip:InfiniteLoop@example.com SIP/2.0
Record-Route: <sip:192.0.2.162:5080
;lr;a;t=612f37e7;s=96e09e8e8c93a8c60bf460029847f4b1>
Via: SIP/2.0/TCP 192.0.2.162
;branch=z9hG4bK-42e47bba67559bd9a3da1934a70bbc37
Via: SIP/2.0/TCP 192.0.2.162:5080
;branch=z9hG4bK-50d909f1209f7a820de85c7831846330
Via: SIP/2.0/TCP 192.0.2.162
;branch=z9hG4bK-994f162bc179fb75093166fabbfd13c7
Via: SIP/2.0/TCP 192.0.2.162:5080
;branch=z9hG4bK-708842ad6ea22f8fa6e39c503d3d803e
Via: SIP/2.0/TCP 192.0.2.162
;branch=z9hG4bK-50b581a06ca023ebcddbc82c5221149c
Via: SIP/2.0/TCP 192.0.2.14:1084
;branch=z9hG4bK994220327571023745d7996c13560a11.0
Via: SIP/2.0/TCP 192.0.2.14:40221
;branch=z9hG4bK-931ea14405e9da8c95cf4ed60a71f59f
To: sip:9999@example.com
From: Sip Send <sip:sipsend@10.1.1.20>;tag=612f37e7
Call-Id: 7a26fdad2cb40d48e81e10d6fce39825@10.1.1.20
Cseq: 1 INVITE
Max-Forwards: 0
User-Agent: sipsend/0.02
Date: Fri, 14 Oct 2005 15:35:53 GMT
Content-Length: 0
The Warning header in this response identifies the server returning
the error (192.0.2.162:5080). The Via headers of the returned
message/sipfrag body show the path the failed message took. The
returned request line also shows that the target URI has been changed
to the user 'InfiniteLoop'.
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Resending the request with a hop limit one less than before (8),
shows that at that hop the request URI is to user 'LoopForever':
INVITE sip:LoopForever@example.com SIP/2.0
Record-Route: <sip:192.0.2.162:5080
;lr;a;t=4f18a30b;s=a9711e1704ccd5273955589c5fe94745>
Via: SIP/2.0/TCP 192.0.2.162
;branch=z9hG4bK-9b7f69455266f7cccd4ae8a285c0417c
Via: SIP/2.0/TCP 192.0.2.162:5080
;branch=z9hG4bK-b9d3e2aff65e68497849a2609bf8c373
Via: SIP/2.0/TCP 192.0.2.162
;branch=z9hG4bK-a178f4c5a3b8bbf35f979bc6c6d33022
Via: SIP/2.0/TCP 192.0.2.162:5080
;branch=z9hG4bK-3c098b8d58e4b6ce98fca3495263e795
Via: SIP/2.0/TCP 192.0.2.162
;branch=z9hG4bK-e7ceb06ed917d59024c905b3ee60e4cc
Via: SIP/2.0/TCP 192.0.2.14:1085
;branch=z9hG4bK8d685f52450e87da45d7996c13560a11.0
Via: SIP/2.0/TCP 192.0.2.14:56114
;branch=z9hG4bK-4ae5a563b0cbc76aef7be17115836dea
To: sip:9999@example.com
From: Sip Send <sip:sipsend@10.1.1.20>;tag=4f18a30b
Call-Id: 39106d45526cb5e78bf8dac378e05817@10.1.1.20
Cseq: 1 INVITE
Max-Forwards: 0
User-Agent: sipsend/0.02
Date: Fri, 14 Oct 2005 15:42:21 GMT
Content-Length: 0
Route: <sip:192.0.2.162:5070;transport=tcp;lr>
Reducing the limit one at a time (or starting from 1 and working
forward), a UAC can determine that the InfiniteLoop/LoopForever
forwarding loop exists (in reality, of course, the user names would
rarely be such good hints), and where in the forwarding sequence the
original '9999' was changed to enter the loop.
Without the returned request headers, the 483 response does not help
the request originator (or any proxy administrator on the path)
diagnose why the error has occurred. With it, in this case a
diagnostic application running as a User Agent is able to at least
identify that there is a routing problem and which proxy is
misrouting the request.
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7. IANA Considerations
None.
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8. Security Considerations
The proposed mechanism provides a means by which topology and some
routing information about a set of SIP systems can be discovered.
The mechanism is very similar to that provided for IP routing by the
traceroute tool.
Some systems may not want to expose as much information as is
available in the full set of SIP request headers by returning them in
the error response body. In this case, the system returning the
error should prune the response as recommended in Section 3.1.
It may be possible for an attacker to forge very large messages with
a deliberately low Max-Forwards header field values so that a Server
will send error responses. If those responses are fragemented and
sent on a transport that does not have congestion control, this could
cause a small number more response packets than the attacker sent
requests. The server is allowed to prune the response as recommended
in Section 3.1 to reduce the response size, which reduces the
opportunity for the attacker to generate many fragments. Other than
this, the returned response message is roughly twice the size of the
original request, and gets smaller as the Via and Route headers are
removed in transit, so there is little amplification.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3420] Sparks, R., "Internet Media Type message/sipfrag",
RFC 3420, November 2002.
9.2. Informative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
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Authors' Addresses
Scott Lawrence
Pingtel Corp.
400 West Cummings Park
Suite 2200
Woburn, MA 01801
USA
Phone: +1 781 938 5306
Email: slawrence@pingtel.com
Alan Hawrylyshen
Ditech Networks Inc.
1167 Kensington Rd NW
Suite 200
Calgary, Alberta T2N 1X7
Canada
Phone: +1 403 806 3366
Email: ahawrylyshen@ditechnetworks.com
Robert Sparks
Estacado Systems
17210 Campbell Road
Suite 250
Dallas, Texas 75254-4203
USA
Email: RjS@nostrum.com
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